In an aircraft electrical generating system the output voltage of a main channel generator must be regulated within a narrow band of output voltage so as to not damage any of the utilization equipment to which it supplies electric power. In conventional systems, a synchronous wound field generator is used to generate the electric power needed by the utilization equipment. This type of generator is typically used because the output voltage can be regulated by controlling the generator field excitation using a generator control unit (GCU). As more electrical loads are connected to the system, the amount of excitation provided to the wound field is increased, thereby preventing an unacceptable droop in output voltage. Such a generator is constructed with a fixed exciter field winding which is driven by the GCU. The excitation provided to this fixed winding induces a voltage in a three phase rotating exciter winding located on the rotor. The output of these rotating windings are rectified by a rotating rectifier assembly to produce a dc output used to drive the rotating main field winding. This main field winding, in turn, excites the main output stator windings which supply the electric power to the utilization equipment.
A problem associated with the use of such a generator is that there are undesirable losses in the rotor resulting from the "inside-out" nature of the exciter field. These losses occur at various points between the fixed winding which inductively couples a rotating three phase winding assembly whose output is rectified by a rotating diode assembly to drive a rotating main field winding which inductively couples the main stator windings. In addition to the lower efficiency resulting from these losses, the overall machine is less robust due to stresses placed on the rotating diode assembly. The use of suppression resistors are required for the rotating rectifier assembly to protect the diodes from large voltage spikes resulting from switching and large load transients.
A more robust and efficient machine is the permanent magnet generator (PMG). This type of generator uses strong permanent magnets constructed from materials such as samarium-cobalt or Nd-Fe-B (where the rotor temperature remains under 100.degree. C.) to generate the excitation flux needed to generate electricity. These permanent magnets are placed on the rotor of the PMG and provide a fixed excitation at a given speed. The absence of the rotating windings and diode assemblies in these PMGs makes them inherently rugged, efficient, and reliable machines. The output voltage of the PMG is a function of load and operating speed of the machine. A problem inherent with such a machine, however, is that the output voltage droops rather severely at a given speed as load is connected to its output as illustrated by the graph of output voltage versus load current 100 in FIG. 1. This is because the fixed excitation from the permanent magnets does not change. One way to counteract this severe voltage droop problem is to change the speed at which the rotor of the PMG is driven. As the speed of the PMG is increased, the PMG output voltage is increased to counteract the drooping voltage. As electrical load is removed from the PMG, the speed may be decreased to suppress the rising voltage.
While this input speed/output voltage control scheme would appear to solve the problem associated with changing electrical loads, such a system to date is unable to meet the power quality specifications which govern aircraft electric power generating systems. These specifications, such as MIL-G-21480, DO-160, and MIL-STD-704, require that narrow band voltage regulation be maintained as loads are switched on and off from no load to twice the normal per unit loading as may be seen from the limits 102.sub.U and 102.sub.L for DO-160 and 104.sub.U and 104.sub.L for MIL-STD-704 of FIG. 2. These specifications also require that this voltage regulation be maintained under normal conditions with allowance for very short transient variations as may be seen from limits 106.sub.U and 106.sub.L for generator loading up to rated load, limits 108.sub.U and 108.sub.L for generator overloading up to 150% rated capacity, and limits 110.sub.U and 110.sub.L for generator loading up to 200% rated capacity. Under such loading conditions it becomes difficult to overcome the rotational inertia of the PMG's rotor to force a speed change quick enough to meet the power quality specifications for the output voltage, not to mention the possibility of shearing the shaft from the torque required to effect such a speed change.
This problem is compounded even further if the electrical system is required to utilize a hydraulic motor to drive the PMG. For this type of system, in addition to the PMG rotor inertia delaying the output voltage response to a speed change command, delays caused by valve openings and closings and the change in hydraulic fluid flow rate further delay the electrical response of the system. These delays may result in the system being tripped off-line by the system's over voltage protection during normal off load transients because the PMG's output voltage increases beyond the trip level for longer than allowed by specification. This type of situation is unacceptable, and his severely limited the application of PMGs for hydraulically driven, highly regulated systems.
It is, therefore, an objective of the instant invention to overcome these and other problems known in the art. Specifically, it is an objective of the instant invention to provide an electric power generating system which is capable of utilizing the rugged and efficient permanent magnet generator. It is further an objective of the instant invention to provide a system which controls the output of the PMG in such a manner as to allow its use in accordance with industry power quality standards. A further objective of the instant invention is to provide a PMG based electrical power generating system which is driven by hydraulic power. Additionally, it is an objective of the instant invention to provide a system which will protect against faults which could damage the utilization equipment or the PMG itself. It is a further objective of the instant invention to provide such a system using a simple and economical control scheme.